TWI521410B - Apparatus and method for acquiring object image of a pointer - Google Patents

Description

Object image capturing device and method for capturing object image of indicator

The present invention relates to an optical touch technology, and in particular, to an object image capturing device and a method for capturing an image of an object of an indicator.

1 is a perspective view of a conventional optical touch system. Referring to FIG. 1 , the optical touch system 100 includes an object image capturing device 101 , a panel 104 , and reflective elements 112 - 116 . The object image capturing device 101 further includes image sensing devices 106 and 108 and a processing circuit 110. The image sensing devices 106 and 108 are used to sense the image of the touch surface 118 on the front panel 104. The processing circuit 110 is electrically coupled to the image sensing devices 106 and 108 to receive images sensed by the two image sensing devices. In this example, the touch surface 118 has a quadrangular shape, preferably a rectangular shape. As for the above-mentioned reflective elements, the light is reflected to the touch surface 118, but neither of them forms a mirror image of the touch surface 118. When the pointer 102 is adjacent to the touch surface 118, the processing circuit 110 obtains the position of the pointer 102 according to the image sensed by the two image sensing devices.

FIG. 2 is an explanatory diagram of the single touch of the optical touch system 100. In FIG. 2, the same reference numerals as those in FIG. 1 are denoted by the same members. As shown in FIG. 2, image sensing device 106 can sense indicator 102 along sensing route 202, while image sensing device 108 can sense indicator 102 along sensing route 204. Therefore, the processing circuit 110 can obtain the linear equation of the sensing route 202 according to the image sensed by the image sensing device 106, and obtain the linear equation of the sensing route 204 according to the image sensed by the image sensing device 108. The processing circuit 110 can then calculate the intersection of the sensing routes 202 and 204 to further calculate the coordinates of the indicator 102 based on the intersection.

Before calculating the coordinates of the indicator 102, the processing circuit 110 must first find the imaging range of the indicator 102 in the image sensing window of the image sensing device 106 from the image sensed by the image sensing device 106 (described later). That is, the processing circuit 110 must first capture the object image of the indicator 102 from the image sensed by the image sensing device 106 to further obtain the linear equation of the sensing route 202. At the same time, the processing circuit 110 must also sense the image. The image sensed by the device 108 is used to find the imaging range of the indicator 102 in the image sensing window of the image sensing device 108 (described later), that is, the processing circuit 110 must first be sensed from the image sensing device 108. The image is captured to capture an image of the object of the indicator 102 to further obtain a linear equation for sensing the route 204. This will be further explained below.

Taking the operation between the processing circuit 110 and the image sensing device 106 as an example, before the indicator 102 is adjacent to the touch surface 118, the processing circuit 110 first senses the touch surface 118 through the image sensing device 106 to obtain a An image of the object image containing the indicator 102 and treating the image as a background image. Then, the processing circuit 110 obtains the brightness values of the N brightest pixels in each row of pixels of the background image, and calculates the brightness average value or the total brightness value of the selected N brightest pixels in each row of pixels, thereby forming one of A luminance distribution map, where N is a natural number. This brightness profile is presented in a curved manner because the background brightness is usually not uniform. 3 is an example of a luminance profile obtained from a background image, and any point in the curve shown in the figure is represented as a luminance value of a row of pixels of the background image.

Then, when the indicator 102 is adjacent to the touch surface 118, the processing circuit 110 can obtain an image of the object image containing the indicator 102 through the image sensing device 106. FIG. 4 is a schematic diagram of an image sensed by the image sensing device. In FIG. 4, reference numeral 400 is shown as an image sensing window of image sensing device 106. The white area indicated by the mark 402 is the light reflected by the light reflecting elements 114 and 116, and a bright zone having a high brightness is formed on the image. The bright area 402 is the main sensing area. As for the indicator 404, it is the dark line caused by the indicator 102, which is the image of the object.

After obtaining the image of the object image containing the indicator 102, the processing circuit 110 treats the image as a sensing image, and obtains the brightness distribution of the sensing image by the same method as obtaining the brightness distribution map. . Figure 5 depicts another brightness profile as described. In FIG. 5, the curve indicated by the indication 502 is a luminance distribution map obtained from the sensing image, and any point in the curve is represented as a luminance value of a row of pixels of the sensing image. The range indicated by the mark W ₁ is the low brightness range caused by the indicator 102 shielding the light. The curve indicated by the flag 504 is a threshold value obtained by the processing circuit 110 from the background image (as shown in FIG. 3) according to a predetermined percentage.

With continued reference to FIG. 5, after the luminance profile 502 is obtained, the processing circuit 110 compares the luminance profile 502 with the threshold 504 to cause the luminance profile 502 to have a luminance value below the threshold 504 (indicating W). refers to the range of ₁₎ the distribution range corresponding to the row of pixels, the imaging range of the image considered sensing window indicator in the image sensing device 102 106. 400. In other words, the processing circuit 110 captures the image information of the imaging range W ₁ as the object image of the indicator 102. In this manner, the processing circuit 110 can further obtain the linear equation of the sensing route 202 according to the imaging range, for example, calculate the center of gravity of the imaging range to further obtain the linear equation of the sensing route 202. Similarly, the operation between the processing circuit 110 and the image sensing device 108 can also be performed according to the operation between the processing circuit 110 and the image sensing device 106 to further obtain the linear equation of the sensing route 204.

However, the optical touch system 100 often has problems when performing multi-touch. Taking the operation between the processing circuit 110 and the image sensing device 106 as an example, when two indicators 102 touch the touch surface 118 and the two indicators 102 are relatively close to each other, the processing circuit 110 A brightness profile is obtained from the sensed image. Figure 6 is a graph showing the brightness distribution. In FIG. 6, the curve indicated by the indication 602 is the brightness distribution map obtained from the sensing image, and any point in the curve is represented as the brightness value of the line pixel of the sensing image. The range indicated by the mark W ₂ is the low brightness range caused by the two indicators 102 shielding the light. The curve indicated by the flag 504 is a threshold value, and the threshold value 504 is obtained by the processing circuit 110 from a background image by a predetermined percentage.

As can be seen from FIG. 6, when the threshold 504 is set too high, the processing circuit 110 treats the two indicators 102 as the same indicator. Therefore, the processing circuit 110 cannot further calculate the coordinates of the two indicators 102.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an object image capturing device that accurately captures an image of an object of a plurality of indicators.

Another object of the present invention is to provide a method of capturing an image of an object of an indicator.

The invention provides an object image capturing device, which is suitable for an optical touch system for capturing an image of an object of an indicator when an indicator interacts with a touch surface of the optical touch system. The object image capturing device comprises an image sensing device and a processing circuit. The image sensing device is configured to sense an image of the touch surface. The processing circuit is coupled to the image sensing device. When the indicator is adjacent to the touch surface, the processing circuit obtains a sensing image through the first image sensing device, and compares at least part of the information in the sensing image with the first threshold to find a Alignment range. In addition, the processing circuit generates a second threshold according to the image information in the comparison range, so as to compare the image information and the second threshold in the comparison range to capture the object image of the indicator.

In a preferred embodiment of the object image capturing device of the present invention, at least part of the information processing circuit obtains brightness values of N brightest pixels in each row of pixels of the sensing image, and calculates each A luminance average or a total luminance value of the selected N brightest pixels in the row of pixels, thereby forming a first luminance profile, where N is a natural number.

In a preferred embodiment of the image capturing device of the object of the present invention, wherein the comparison range is the range covered by all the line information in which the brightness value is lower than the first threshold value in the first brightness distribution map.

In a preferred embodiment of the object image capturing device of the present invention, the second threshold distribution obtained by the first threshold value processing circuit from a background image is obtained according to the first predetermined percentage. The background image processing circuit senses the touch surface through the image sensing device before the indicator is adjacent to the touch surface, and thus obtains an image of the object image without the indicator in advance. The second brightness profile is also obtained by the processing circuit by calculating the brightness average value or the total brightness value of the N brightest pixels selected in each row of pixels of the background image.

In a preferred embodiment of the object image capturing device of the present invention, the processing circuit obtains the second threshold value from the second brightness distribution map according to the second predetermined percentage.

In a preferred embodiment of the image capturing device of the object of the present invention, wherein the processing circuit obtains a lowest point from a curve segment corresponding to the comparison range of the first brightness distribution map, and the lowest point The luminance value is the reference and the predetermined luminance is further increased to obtain the second threshold.

In a preferred embodiment of the image capturing device of the object of the present invention, the image of the object capturing the indicator includes image information that is smaller than the second threshold in the comparison range as the object image.

The invention further provides a method for capturing an image of an object of an indicator, which is suitable for an optical touch system. The optical touch system includes a touch surface and an image sensing device. The image sensing device is configured to sense an image of the touch surface. The method includes the following steps: when an indicator is adjacent to the touch surface, the image sensing device is used to obtain a sensing image, and at least part of the information in the sensing image is compared with the first threshold value to Finding a comparison range; generating a second threshold according to the image information in the comparison range; and comparing the image information and the second threshold in the comparison range to capture the object image of the indicator.

In a preferred embodiment of the method of the present invention, at least part of the information is obtained by taking the brightness values of the N brightest pixels in each row of pixels of the sensing image, and calculating the selected one of each row of pixels. The average brightness value or the total brightness value of the N brightest pixels, thereby forming a first brightness profile, where N is a natural number.

In a preferred embodiment of the method of the present invention, wherein the comparison range is the range covered by all the row information in which the luminance value in the first luminance profile is lower than the first threshold.

In a preferred embodiment of the method of the present invention, the first threshold value obtained by the first threshold is obtained according to the first preset percentage. The background image senses the touch surface through the image sensing device before the indicator is adjacent to the touch surface, thereby obtaining an image of the object image without the indicator in advance. The second brightness profile is also obtained by calculating the brightness average value or the total brightness value of the N brightest pixels selected in each row of pixels of the background image.

In a preferred embodiment of the method of the present invention, the second threshold value is obtained according to the second predetermined percentage through the second brightness profile.

In a preferred embodiment of the method of the present invention, a lowest point is obtained from a curve segment corresponding to the comparison range of the first brightness distribution map, and the brightness value of the lowest point is used as a reference. Increase the predetermined brightness to obtain the second threshold.

In a preferred embodiment of the method of the present invention, the image of the object that captures the indicator includes image information that is smaller than the second threshold in the comparison range as the object image.

The optical touch system of the present invention utilizes two different threshold values to find an object image (ie, an actual imaging range) of a plurality of indicators. In the actual operation mode, the processing circuit first uses the first threshold value to find a rough imaging range of the plurality of indicators in the image sensing window of the image sensing device, and the approximate imaging range needs to be further compared. Alignment range. Then, the processing circuit generates a second threshold according to the image information in the comparison range, so as to compare the image information and the second threshold in the comparison range, and then extract the object image of the indicators, that is, find out The actual imaging range of these indicators. In this way, the coordinates of the above indicators can be further calculated according to the actual imaging range.

Therefore, as long as the size of the second threshold is properly designed, the processing circuit can accurately find the object images of the indicators, and then calculate the actual coordinates of the indicators.

The above and other objects, features and advantages of the present invention will become more <RTIgt;

The hardware structure of the optical touch system of the present invention is the same as that of the optical touch system shown in FIG. 1, but the difference between the two is the object image of the optical touch system of the present invention. The device is taken in another way. Therefore, the following operation mode of the optical touch system of the present invention will be described using the hardware architecture shown in FIG.

Please refer to Figure 1. As shown in FIG. 1 , the touch surface 118 has four sides (not labeled) connected in sequence, and the image sensing devices 106 and 108 are disposed at two different corners of the touch surface 118 and are located on the touch surface. The same side of 118. As such, image sensing devices 106 and 108 can sense images of touch surface 118 from two different angles.

Next, a multi-touch method of the optical touch system of the present invention will be described. For example, before the operation of the processing circuit 110 and the image sensing device 106, the processing circuit 110 first senses the touch surface 118 through the image sensing device 106 before any indicator 102 is adjacent to the touch surface 118. An image of the object image without the indicator 102 is obtained and used as a background image. Then, the processing circuit 110 obtains the brightness values of the N brightest pixels in each row of pixels of the background image, and calculates the brightness average value or the total brightness value of the selected N brightest pixels in each row of pixels, thereby forming a brightness. Distribution map, where N is a natural number.

Then, when two indicators 102 are adjacent to the touch surface 118, and the two indicators 102 are relatively close to each other, the processing circuit 110 can obtain the objects including the two indicators 102 through the image sensing device 106. Image of the image. After the image of the object image containing the two indicators 102 is obtained, the processing circuit 110 treats the image as a sensing image. Of course, the sensing image is a sensing image containing infrared brightness information. Then, the processing circuit 110 compares at least part of the information in the sensing image with the first threshold to find a rough imaging range of the two indicators 102 in the image sensing window of the image sensing device 106. However, this outline imaging range is a range of comparisons (further described later) that require further comparison. In this example, at least part of the information processing circuit 110 obtains the brightness values of the N brightest pixels in each row of pixels of the sensing image, and obtains the same method as the method for obtaining the brightness distribution map. A brightness profile. The brightness distribution map obtained by the threshold value processing circuit 110 from the background image is obtained according to a predetermined percentage. Figure 7 depicts another brightness profile as described.

In FIG. 7, the curve indicated by the indication 602 is the brightness distribution map obtained from the sensing image, and any point in the curve is represented as the brightness value of the line pixel of the sensing image. The range indicated by the mark W ₂ is the low brightness range caused by the two indicators 102 shielding the light. The curve indicated by the indication 504 is the first threshold. The role of the dotted line indicated by the indication 702 will be detailed later. It can be seen from FIG. 7 that the low brightness range indicated by W ₂ is the approximate imaging range of the two indicators 102 in the image sensing window of the image sensing device 106, that is, the range of comparisons that need to be further compared. The comparison range is the range covered by all the line information in the brightness distribution map of the sensing image whose brightness value is lower than the first threshold value. In other words, the processing circuit 110 is the luminance profile 602 compared with the first threshold value 504, and the luminance value of the luminance distribution section 602 is lower than the first threshold 504 (within the meaning of W ₂ Flag The distribution range of the corresponding row pixels is regarded as the first imaging range of the two indicators 102 in the image sensing window 400 of the image sensing device 106. This first imaging range is the approximate imaging range of the two indicators 102.

Referring to FIG. 7 , after obtaining the foregoing comparison range, the processing circuit 110 generates a second threshold according to the image information in the comparison range, so as to compare the image information and the second threshold in the comparison range. The actual imaging range of these indicators 110 is then found. The manner in which the second threshold is generated will be further explained below.

In this embodiment, the processing circuit 110 corresponds to line 602 of FIG obtain a lowest point of the range in the ratio (i.e., the first imaging range) of the curve segment (i.e., curve segment in W in the range of ₂₎ from the luminance distribution. Since point A and point B in this example are both the lowest point, the processing circuit 110 takes a point in these two points. Then, the processing circuit 110 resets the second threshold between the first threshold 504 and the corresponding brightness of the lowest point (point A or point B). In this example, the processing circuit 110 obtains the second threshold by adding a predetermined brightness based on the total luminance of the row pixels corresponding to the lowest point, and the second threshold is as shown in FIG. 7. Indicated at 702.

After the second threshold 702 is obtained, the processing circuit 110 compares the curve segment corresponding to the comparison range and the second threshold 702 of the luminance profile 602 for capturing the object images of the two indicators. In this example, the processing circuit 110 is regarded as the distribution range of the row pixels corresponding to the portion of the curve segment whose total luminance value is lower than the second threshold value 702, and is regarded as the two indicators 102 in the image sensing device 106. The second imaging range in the image sensing window 400. This second imaging range is the actual imaging range of the two indicators 102. In other words, the processing circuit 110 can obtain two curve segments lower than the second threshold value 702 in the luminance profile 602 according to the second threshold value 702, thereby further distributing the row pixels corresponding to the two curve segments. The range is taken as the actual imaging range of the two indicators 102 in the image sensing window 400 of the image sensing device 106. To be concise, the image information smaller than the second threshold value 702 in the comparison range is taken as the object image of the two indicators. In this way, the processing circuit 110 can further obtain the linear equations of the corresponding two sensing routes according to the actual imaging ranges of the two indicators 102.

Similarly, the operation between the processing circuit 110 and the image sensing device 108 can also be performed according to the operation between the processing circuit 110 and the image sensing device 106 to further obtain the straight line equations of the other two sensing routes. Then, the processing circuit 110 can further obtain the coordinates of the two indicators 102 according to the four linear equations. As can be seen from the above description, even if the two indicators 102 are relatively close to each other, the optical touch system of the present invention can accurately find the object images of the two indicators 102 (ie, the actual imaging range), and then calculate The actual coordinates of the two indicators 102. Therefore, the optical touch system of the present invention can accurately perform coordinate positioning of multi-touch.

In this example, the processing circuit 110 obtains the second threshold value 702 (which is a straight line) by adding a predetermined brightness based on the total luminance of the row pixels corresponding to the point A or the point B, but the processing circuit 110 may also be that the luminance profile obtained from the background image acquires another threshold (which is a curve) according to another predetermined percentage in order to replace the luminance threshold 702. Of course, another threshold value used to replace the threshold value 702 would be between point A (or point B) and the first threshold value 504. In addition, the processing circuit 110 can also sense the lowest luminance value of the luminance distribution map of the image in the above comparison range as the second threshold value. In addition, after the processing circuit 110 obtains a luminance distribution map by calculating the luminance values of each row of pixels of the background image, the luminance distribution map can be recorded first, so that it is not necessary to repeat such operations repeatedly. In addition, the processing circuit 110 may obtain the background image through one of the image sensing devices, and then obtain the sensing image through the other image sensing device.

In addition, with the above teachings, those skilled in the art should know that even if the object image capturing device 101 includes only one image sensing device and one processing circuit, the object image capturing device 101 can perform the above-mentioned obtaining indicator. The operation of the object image, and can accurately capture the image of each object of the plurality of indicators. It is worth mentioning that each of the reflective elements of this example can be made with a retro-reflective material for better results. In addition, each of the retroreflective elements of this example can be replaced by a light-emitting element, as long as each of the light-emitting elements emits light toward the touch surface 118.

FIG. 8 illustrates an image sensing device suitable for use in the optical touch system of the present invention. Referring to FIG. 8 , the image sensing device 800 includes an infrared (IR) illumination device 802, an infrared filter device 804 that allows only infrared rays to pass through, and a photosensor 806. The optical sensor 806 is configured to obtain an image of the touch surface 118 through the infrared filter 804 and is coupled to the processing circuit 110. Further, the infrared illuminating device 802 can be realized by an infrared light emitting diode (IR LED), and the infrared ray filtering device 804 can be realized by an infrared filter (IR-pass filter).

With the above teachings, a method of capturing an image of an object of an indicator can also be summarized, as shown in FIG. FIG. 9 is a flow chart of a method for capturing an image of an object of an indicator according to an embodiment of the invention, which is applicable to an optical touch system. The optical touch system includes a touch surface and an image sensing device. The image sensing device is configured to sense an image of the touch surface. The method includes the following steps: when an indicator is adjacent to the touch surface, the image sensing device is used to obtain a sensing image, and at least part of the information in the sensing image is compared with the first threshold value to find a comparison range (as shown in step S902); generating a second threshold according to the image information in the comparison range (as shown in step S904); and comparing the image information and the second threshold in the comparison range And an image of the object for capturing the indicator (as shown in step S906).

In summary, the optical touch system of the present invention utilizes two different threshold values to find an object image (ie, an actual imaging range) of a plurality of indicators. In the actual operation mode, the processing circuit first uses the first threshold value to find a rough imaging range of the plurality of indicators in the image sensing window of the image sensing device, and the approximate imaging range needs to be further compared. Alignment range. Then, the processing circuit generates a second threshold according to the image information in the comparison range, so as to compare the image information and the second threshold in the comparison range, and then extract the object image of the indicators, that is, find out The actual imaging range of these indicators. In this way, the coordinates of the above indicators can be further calculated according to the actual imaging range.

Therefore, as long as the size of the second threshold is properly designed, the processing circuit can accurately find the object images of the indicators, and then calculate the actual coordinates of the indicators.

While the present invention has been described in its preferred embodiments, the present invention is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

100. . . Optical touch system

101. . . Object image capturing device

102. . . Indicator

104. . . panel

106, 108, 800. . . Image sensing device

110. . . Processing circuit

112~116. . . Reflective element

118. . . Touch surface

202, 204. . . Sensing route

400. . . Image sensing window

402. . . Bright area

404. . . Dark grain

502, 602. . . Brightness map

504, 702. . . Brightness threshold

802. . . Infrared lighting device

804. . . Infrared filter

806. . . Light sensor

A, B. . . point

S902~S906. . . step

W ₁ , W ₂ . . . Low brightness range

1 is a perspective view of a conventional optical touch system.

FIG. 2 is an explanatory diagram of a single touch of a conventional optical touch system.

3 is an illustration of a luminance profile obtained from a background image.

4 is a schematic diagram of an image sensed by an image sensing device.

Figure 5 depicts another luminance profile.

Figure 6 depicts another luminance profile.

Figure 7 depicts another luminance profile.

FIG. 8 illustrates an image sensing device suitable for use in the optical touch system of the present invention.

9 is a flow chart of a method of capturing an image of an object of an indicator in accordance with an embodiment of the present invention.

S902~S906. . . step

Claims (18)

An object image capturing device is applicable to an optical touch system for capturing an object image of the indicator when an indicator interacts with a touch surface of the optical touch system, including: an image sensing device And the processing circuit is coupled to the image sensing device. When the indicator is adjacent to the touch surface, the processing circuit obtains a sensing image through the image sensing device. And comparing at least part of the information in the sensing image with a first threshold value to find a range of the at least part of the information in the sensing image, and according to the comparison range The image information generates a second threshold value, and compares the image information in the comparison range with the second threshold value, and uses the image information in the comparison range that is smaller than the second threshold value to obtain the object of the indicator object. image. The object image capturing device of claim 1, wherein at least part of the information is that the processing circuit obtains brightness values of N brightest pixels in each row of pixels of the sensing image, and calculates each A luminance average or a total luminance value of the selected N brightest pixels in the row of pixels, thereby forming a first luminance profile, where N is a natural number. The object image capturing device of claim 2, wherein the comparison range is a range covered by all the line information in the first brightness distribution map whose brightness value is lower than the first threshold value. The object image capturing device of claim 1, wherein the first threshold value is obtained by the processing circuit for obtaining a second brightness profile from a background image according to a first predetermined percentage. The image processing system is configured to sense the touch surface through the image sensing device before the indicator is adjacent to the touch surface, so that the image without the indicator is obtained in advance, and the second brightness distribution map is obtained. It is also the processing circuit that calculates each line of the background image The brightness average or brightness total value of the N brightest pixels selected in the pixel. The object image capturing device of claim 4, wherein the processing circuit obtains the second threshold value from the second brightness distribution map according to a second predetermined percentage. The object image capturing device of claim 1, wherein the processing circuit obtains a lowest point from a curve segment corresponding to the comparison range of the first brightness distribution map, and the lowest point The brightness value is referenced and a predetermined brightness is added to obtain the second threshold value. The image capturing device of the object of claim 1, wherein the image of the object that captures the indicator comprises capturing image information of the comparison range that is smaller than the second threshold as the object image. The object image capturing device of claim 1, further comprising an infrared illuminating device and only one infrared ray filtering device, and the image sensing device is permeable to the infrared ray filtering device Obtain an image of the touch surface. The object image capturing device of claim 1, wherein the second threshold value is between the first threshold value and a lowest point of the curved portion corresponding to the comparison range. A method for capturing an image of an object of an indicator is applicable to an optical touch system, the optical touch system includes a touch surface and an image sensing device, wherein the image sensing device is configured to sense the touch surface The method includes the following steps: when an indicator is adjacent to the touch surface, the image sensing device is used to obtain a sensing image, and at least part of the information in the sensing image is compared with a first threshold value. Comparing to find an alignment range of the at least part of the information in the sensing image; Generating a second threshold according to the image information in the comparison range; and comparing the image information in the comparison range with the second threshold to capture an image of the comparison range that is smaller than the second threshold Information to obtain an image of the object of the indicator. The method of claim 10, wherein at least part of the information is obtained by obtaining luminance values of N brightest pixels in each row of pixels of the sensing image, and calculating a selected N of each row of pixels. The brightness average or brightness total of the brightest pixels, which in turn forms a first luminance profile, where N is a natural number. The method of claim 11, wherein the comparison range is a range covered by all line information in the first brightness distribution map whose brightness value is lower than the first threshold value. The method of claim 10, wherein the first threshold value is obtained by a second brightness profile obtained by using a background image according to a first predetermined percentage, the background image being at the indicator Before the touch surface is sensed, the touch surface is sensed by the image sensing device, so that the image without the indicator is obtained in advance, and the second brightness distribution map is also calculated by calculating each line of the background image. The brightness average or brightness total value of the N brightest pixels selected in the pixel. The method of claim 13, wherein the second threshold value is obtained according to a second predetermined percentage through the second brightness profile. The method of claim 10, wherein a lowest point is obtained from a curve segment corresponding to the comparison range of the first brightness profile, and is further increased based on a brightness value of the lowest point. A predetermined brightness is used to obtain the second threshold. For example, the method described in claim 10, wherein the finger is extracted The object image of the object includes image information that is less than the second threshold value in the comparison range as the object image. The method of claim 10, wherein the sensing image is a sensing image containing infrared brightness information. The method of claim 10, wherein the second threshold value is between the first threshold value and a lowest point of the curve segment corresponding to the comparison range.